Electrolytic material removal wherein the charge in the electrolyte is partially dissipate



Sept. 24, 1968 ELECTROLYTIC R w mz ma 9 M United States Patent 3,403,085ELECTROLYTIC MATERIAL REMOVAL WHEREIN THE CHARGE IN THE ELECTROLYTE ISPAR- TIALLY DISSIPATE Elmer Joseph Berger, Cincinnati, and David ClemensPerin, Madeira, Ohio, assignors to General Electric Company, acorporation of New York Filed Dec. 20, 1965, Ser. No. 515,296 6 Claims.(Cl. 204-143) This invention relates to electrolytic material removaland, more particularly, to an improved electrolytic method especiallyuseful in the production of small holes in an electrically conductivearticle.

In co-pending application Ser. No. 474,833 filed July 26, 1965, andassigned to the assignee of the present invention, a method andapparatus for electrolytic material removal is described. In oneembodiment that method directs a stream or jet of charged elecrolytefrom a nozzle toward a workpiece, the power input of the system beingsufiicient to place the electrolyte contacting the workpiece in acondition wherein the current-voltage relationship in the electrolyte isin the Kellogg region (infra) and is manifested at least as an incipientglow through a well defined glow in the electrolyte.

As used in this specification, like the above identified co-pendingapplication, the term incipient glow in the electrolyte refers to thecondition in the electrolyte between a cathode and an anode at which thecurrent density is greater than that which produces the quiet evolutionof bubbles in direct current electrolysis. It is in the range of thetransition region reported by Kellogg in the Journal of ElectrochemicalSociety, 1950, 97, 133- 142. At the point where an incipient glowappears in the electrolyte, the current decreases with increasingvoltage whereas prior to the appearance of an incipient glow in theelectrolyte, current increases linearly with increasing voltage. At thepoint of incipient glow, vapor begins to form at the anode surface andthe formation of the characteristic bubbles normally formed inelectrolysis at that area ceases. The range from this point of powerapplication through the electrolyte up to the level at which a visualglow can be observed is sometimes designated as the Kellogg region andis used herein to refer to the condition in the electrolyte causing atleast incipient glow in the electrolyte as it applies to electrolyticmaterial removal. Further into this region, current is low and generallyconstant as voltage is increased.

In one specific form of the method and apparatus of that co-pendingapplication, a tool which can include means to apply a negative charge,at least part of the time, to the electrolyte system is positionedopposite the workpiece across a space or gap. Such gap is small enoughto allow the electrolyte stream or jet issuing from the the tool andcontacting the workpiece to be placed in a condition causing at least anincipient glow in the electro-' lyte. Other operating conditions andmaterials being equal, the gap between the tool, and thus the means toapply the negative charge, and the workpiece will control the size andshape of the cavity resulting from removal of material by this methodand apparatus. As one practical method of sensing and controlling thisgap, the co-pend-.

ing application describes a method for decreasing the starting spacebetween the tool and workpiece at a first rate until a sensing meansrecognizes an increase, to a certain value, in current flow as afunction of gap. Then, when the workpiece and tool are substantially attheir operating spacing, the feed rate is reduced to feed the workpieceand tool more slowly one toward the other at an operating feed rate forthe actual material removal operation based on the materials andconditions used in the method. In this way, the tool nozzle, whichguides Patented Sept. 24, 1968 or directs the electrolyte and which isfrequently vmade of glass, is protected from damage. More important,however, the size and shape of the cavity produced is accuratelycontrolled.

Another sensing and control means required in the practice of onespecific embodiment in the above identified copending application forcomplete penetration of the workpiece is means to sense firstbreakthrough of the workpiece and the associated drop in electriccurrent flow because of the smaller contact area and greater resistance.Under such breakthrough conditions, the feed rate is further reduced orin some cases actually stopped in the practice of that embodiment toallow the complete penetration of the workpiece to be accomplishedsmoothly.

The apparatus required to sense variations in current at severalselected times in the specific embodiment of the process discussedabove, and as a result to control and vary feed rate as a function ofcurrent and gap, employs fairly complex circuitry and a variety ofnumerous interrelated components. Many of these components are bestoperated, preset or monitored by an equipment operator for eachworkpiece.

A principal object of the present invention is to provide an improvedmethod for conducting electrolytic material removal of the typedescribed in the above identified co-pending application but which doesnot require gap sensing at various times in the method and eliminatesthe need for variable speed feed mechanisms.

Another object is to provide an improved, efficient and practical methodfor the concurrent production of multiple small holes in a complexshaped workpiece.

These and other objects and advantages will be more clearly understoodfrom the following detailed description, examples and the drawing all ofwhich are meant to be exemplary of the invention rather than anylimitation on the scope of the present invention.

In the drawing:

FIG. 1 is a sectional view of an article processed by the presentinvention;

FIG. 2 is a partially sectional, partially diagrammatic view ofapparatus for the practice of this invention;

FIGS. 3, 4 and 5 are fragmentary, sectional views of several stages ofmaterial removal;

FIGS. 6 and 7 are fragmentary, sectional views of a comparison betweenentry holes as a function of starting gap.

The method of the present invention provides an automatic gap controland hence a means for eliminating complex variable feed means and gapsensing means, for example, as a function of current, at various pointsin the above described process. This method controls feed rate byproviding an electric conducting means, one example of which is anelectrolyte bath, surrounding the charged electrolyte stream. Such meansdissipates the electric charge from the electrolyte stream between thecathode and the workpiece at gaps or distances at which material removalwould be inacurrate or undesirable. This gap, most efficient formaterial removal, is hereinafter called the equilibrium gap. The size ofthe equilibrium gap is a function of the amount of electric charge inthe electrolyte stream and the ability of the electric charge conductingmeans, such as an electrolyte bath, to dissipate the electric chargefrom the charged electrolyte stream. Across the equilibrium gap currentis allowed to flow toward the workpiece to remove material at about thesame linear rate at which the cathode and workpiece are moved one towardthe other.

The electrical conducting characteristics of the electrolyte stream andof the electric conducting means, such as the electrolyte bath, can bemade constant by fixing such variables as applied voltage, temperatures,pressures, size of electrolyte stream, etc. Therefore the equilibriumgap between the cathode and workpiece across which material removalbegins need not be sensed or otherwise controlled. Thus a constant feedrate can be used for a single tool or a plurality of tools from whichcharged electrolyte is directed. Furthermore, because an electricalconductor such as the electrolyte bath surrounds the workpiece surface,added resistance to current flow is eliminated when a hole is first madethrough a workpiece. Hence no change in feed rate is required for suchoperation.

The improved electrolytic method of this invention for removing materialfrom an electrically conductive workpiece includes the steps ofdirecting a stream of electrically charged electrolyte from a cathodetoward the workpiece across an equilibrium gap while at the same timeapplying between the cathode and the workpiece an electrical potentialof a least 300 volts and passing an electrical current sufficient toproduce in the electrolyte stream contacting the workpiece a conditioncausing at A least an incipient glow in the electrolyte as described inthe above identified co-pending application. Concurrently, theelectrolyte stream is contacted with an electric charge conducing means,such as an electrolyte bath, which will dissipate a sufficient amount ofcharge to inhibit electrolytic material removal of the workpiece at gapsgreater than the equilibrium gap. This inhibition persists until anequilibrium gap is reached if there is relative movement between thecathode and the workpiece.

The above identified co-pending application describes in detail theunusual benefit of maintaining the electrolyte, which passes between thecathode and contacts the workpiece, a condition causing at least anincipient glow in the electrolyte and the wide variety of materialswhich can be processed. It has been found that a potential of at leastabout 300 volts, more particularly in the range of 300 1200 volts andpreferably 400-800 volts is desired when producing small diametercavities or holes such as of about 0.05 or less as a maximum widthdimension. In particular, that co-pending application describes thesignificantly improved rate of material removal which can be obtained bymaintaining the electrolyte contacting the workpiece a condition causingat least an incipient glow in the electrolyte as well as conditionscausing a well defined, clearly visible glow in the electrolyte.However, the power input must be held at a level less than that whichproduces a workpiece-enveloping vapor film which increases electricalresistance between the cathode and the workpiece sufi'icient toterminate removal of material from the workpiece.

The present invention will be more full understood with reference to thedrawing and the following examples as typical embodiments of theinvention. Although the drawings are shown to include a plurality ofstreams of charged electrolyte directed toward a workpiece, it should beunderstood that the present invention can be used with a single streamas well. The present invention, however, is particularly significant inthe concurrent production of a plurality of cavities or holes in acomplex shaped workpiece because of the automatic gap control providedby the electric conducting means such as the electrolyte bath. Thus aplurality of electrolyte directing means or nozzles from each of whichis directed a charged electrolyte stream can be located at about thesame or a variety of levels or positions with respect to the workpiecewithout affecting the subsequent production of the cavities or holes inthe workpiece or the quality and entrance angles of such holes orcavities. Because the electrolyte stream and the electric conductingmeans such as the electrolyte together control operating gap, the needfor accurate mechanical gapping is eliminated.

The present invention can be used to produce a variety of holes orcavities in a variety of articles. Such articles include tubes, bars,plates and other members such as the fluid spray head shown generally atin FIG. 1. For

simplicity in the drawing, the perforations or holes 12 through sprayhead 10 are shown to lie in the same cross sectional plane. It will beunderstood, however, that the present method can be used to place holesat any point in an article in the same or a variety of cross sectionalplanes. As will be described in connection with the electrolyte manifold16 in FIG. 2, the nozzles 14 from which'are directed the chargedelectrolyte stream 26 can protrude in random orientation from the faceof such manifold in order to locate the holes wherever desired in theworkpiece.

Referring to FIG. 2, the workpiece 10, supported by an electricalinsulating means 11, is immersed in an electrical conducting means inthe form of an electrolyte bath 18 at least to the extent that the bathwill cover the portion of the workpiece surface into which or throughwhich holes or cavities, shown by dotted lines, are to be placed. Theelectrolyte bath 18 will dissipate, at least a portion of the chargefrom electrolyte stream 26 directed toward the workpiece from open tip15 of nozzle 14 to tool 13. Each of the plurarity of tools and theirnozzles protruding from the manifold 16 includes, in this embodiment, acathode 20 from which the electrolyte stream gets its charge although insome instances it may be desirable to use a common cathode.

Workpiece 10, and from it electrolyte bath 18, is made to be anodic withrespect to cathode 20 and hence anodic with respect to the electrolytestream 26 directed from tip 15 of nozzle 14. In FIG. 2, manifold 16 ismovable at least toward and away from workpiece 10. Preferably it ismovable in a three dimensional mariner by a conventional motionproducing machine such as a press, not shown, but representedschematically by arrows 28. Electrolyte is supplied to manifold 16 fordistribution through nozzles 14 by pump 22 from electrolyte supply tank24.

The temperature of the electrolyte in tank 24 and of the electrolytebath 18 each can be controlled by a heating or cooling meansdiagrammatically represented at 30 and 32 respectively. This temperaturecontrol is a factor in the adjustment of the relative conductivities ofthe charged electrolyte issuing from nozzle 14 and the electrolyte bathwhich acts to dissipate at least a portion of the charge from theelectrolyte stream 26 until a desired operating gap has been reached. Bymaintaining constant operating conditions, the size of the holes orcavities to be produced in workpiece 10 can be made accurately anduniformly. Their size and shape are controlled without mechanically orelectrically sensing the gap between the tip 15 of nozzle 14 and theworkpiece, which gap relates to the distance between cathode 20 and theworkpiece 10.

Cathode 20 for the plurality of nozzles 14 is connected as a cathode toa source of electrical power such as a DC. rectifier which is capable ofsupplying at least 300 volts. A broadly useful type would supply acurrent of from a small amount which will allow material removal from apoint at which an incipient glow appears in the electrolyte up to about4 amps, although up to about 2 amps would be sufiicient for mostoperations. For certain types of operations, it is possible to connectcathodes 20 with an alternating current source of at least 300 volts.Tools 13 are located in manifold 16 in a pattern which relate tips 15 ofnozzles 14 to the pattern of holes or cavities desired in the workpiece10. The capillary portion of nozzle 14 is made sulficiently long, suchas D in FIG. 1, to penetrate a desired distance into or through theworkpiece.

As was mentioned above, no variation in nozzle feed rate is required aseach hole breaks through a workpiece such as to the internal portion ofthe spray head 10 in the drawing. Therefore, all of the nozzles can bemoved at the same time and at the same rate toward the workpieceirrespective of whether or not they are first starting to removeworkpiece material or are in an intermediate position within theworkpiece, or are completing penetration of the workpiece. If thenozzles are of the same size and extend the same length from manifold 16as shown in FIG. 2, the capillary portion of nozzle 14 should besufiiciently long as shown by D in FIG. 1 to allow the cavity or holeinitially farthest from the manifold to be completed without having thetransition or enlarged area beginning at about 36 to contact theworkpiece.

In another arrangement, the enlarged portion of tool 13 in which thecathode has been located in the drawing can be adjusted in length inorder to eliminate the need for making excessively long capillaryportions. Thus the embodiment shown in FIG. 2 could be modified so thatthe portions of the tools carrying the cathode could be madeprogressively longer from the center tools outwardly. Because there isno critically with regard to gap sensing, since the electrolyte streamand the electrolyte bath control gap, the tips 15 of nozzles 14 can belocated relatively inacurrately and non-uniformly with regard to thedistance between the nozzle tip and the workpiece.

Prior to operation, an electrolyte is selected for the material of theworkpiece. Also, the temperature and pressure at which a desired holewill be produced is selected for the electrolyte stream in relation tothe temperature of the electrolyte bath 18. A variety of electrolytesused in electrolytic machining and the procedures for selectingoperation conditions have been widely described in the literature and inthe above identified c0- pending application. Two preferred electrolytesfor Ni base alloys are a weight percent sulfuric acid aqueous solutionand a 14 weight percent hydrochloric acid aqueous solution. Mostconvenient to use is an electrolyte bath of the same composition as theelectrolyte stream. In this way, if desired, the electrolyte can berecirculated such as from an outlet 40 in the enclosure 42 back toelectrolyte tank 24, or perhaps through a filter if desired.

With the operating conditions established, including an equilibrium feedrate which is not faster than the rate at which tip 15 of the nozzle 14will penetrate the workpiece, the variables of feed rate, electrolytepressure, electrolyte temperature as well as the voltage applied tocathode can be set and controlled automatically by well knowncomponents. As the manifold is moved toward workpiece 20, with theelectrolyte stream 26 issuing from each nozzle 14 and being charged bycathode 20, the electrolyte stream 26 first contacts electrolyte bath 18and then tips 15 of nozzles 14 become immersed in bath 18. Becauseelectrolyte bath 18 is an electric conducting means, at least a part ofthe electric charge is dissipated from the electrolyte stream. However,as shown in FIG. 3, at an equilibrium gap between the workpiece 10 andnozzle 14a sufficient current will flow through the electrolyte stream26a from cathode 20 to workpiece 10 at voltages of at least 300 volts toplace the electrolyte stream in a condition causing at least anincipient glow in the electrolyte. It has been found that at spacinggreater than such gap, the electrolyte bath 18 will inhibit anyelectrolytic material removal. The electrolyte bath is anodic withrespect to the cathode electrolyte stream and appears to the stream as afalse workpiece. Such a larger gap is shown between nozzle 14b andworkpiece 10 in FIG. 3 at which substantially no material removal occursopposite nozzle 14b even though the electrical potential is 300 volts orgreater.

Inaccurate gap control between cathode 20 and work piece 10 whenoperating outside the scope of the present invention such as in air canresult in the funnel shaped entry 46 to the hole in workpiece 10 asshown in FIG. 6. Because in the practice of this invention there is asudden change from no material being removed from the workpiece tomaterial removal at an equilibrium rate, the entrance 48 to the cavityor hole produced by the charged electrolyte stream is very accuratelydimensioned as shown in FIG. 7. Therefore, as the nozzle 14a in FIG. 4,fed by manifold 16, penetrates further into workpiece 10 at the normaloperating feed rate, nozzle 14b can approach the surface of workpiece 10upon which it is to act without causing the funnel shaped entrance asshown in FIG. 6. With further penetration and actual breakthrough of thecharged stream 26a from nozzle 14a as shown in FIG. 5, the feed rateneed not be decreased to complete the full penetration of the hole aswould be required outside the practice of the present invention. Theflow of electrical current through the electrolyte stream at thebreakthrough remains the same because of the surrounding electrolytebath. Consequently the rate of material removal remains the same andnozzle 14b can be fed at the same constant feed rate without concern fornozzle 14a contacting shelf 44. Thus a plurality of holes or cavitiescan be placed in a complex shaped workpiece without varying feed rate ofthe nozzles and without mechanical or electrical sensing and controllingof gap other than by the inherent characteristics of the fixed operatingconditions and of the electrolytes involved.

In a more specific and typical example of actual production linepractice of the method of the present invention, it was desired toproduce 12 holes of about 0.02

diameter in a nickel base alloy tube having a nominal composition, byweight, of 0.1% (max) C; 15% Cr; 3% Cb; 3% Mo; 3% W; 7% Fe; 0.5% A1;0.6% Ti; 0.006% B; with the balance Ni and incidental impurities,sometimes known as IN 102 nickel base alloy. The tube had a wallthickness of 0.0 Accurately dimensioned holes were placed repeatedlythrough both walls of the tube over a distance of about 0.19" throughthe two walls and the central cavity at an applied voltage of 470 voltsand a total of 7.1 ampere current for the 12 tubes. In this example, theelectrolyte used for the electrolyte stream as well as for theelectrolyte bath acting as the electric current dissipating means was anaqueous solution of 10 weight percent sulfuric acid. The temperature ofthe bath and the temperature of the charged electrolyte was maintainedat about F., with an electrolyte pressure of between 40-60 p.s.i. Thiscombination of voltage and current, which produced a condition causingat least an incipient glow in the electrolyte stream, allowed a cuttingrate of 0.06" per minute through the tube wall.

It has been found that for the particular application as shown in theabove example, by varying the temperature of the eletcrolyte between70-200 F. and the applied voltage between 400-600 volts, hole size canbe controlled and varied between 0.02-0.03" for the nozzles used. Inthis example, the nozzle was a drawn glass capillary tube having aninside diameter of 0.015" and a wall thickness of about 0.001".

Although the present invention has been described in connection withspecific examples and embodiments as well as specific conditions, itwill be recognized by those skilled in the art, particularly the art ofelectrochemistry and electrolytic processing, the variations andmodifications of which this invention is capable. By the appendedclaims, it is intended to cover all such equivalent variations andmodifications.

What is claimed is:

1. An electrolytic method for removing material from an anodic workpiecethrough the use of a hollow tool having a dielectric wall encasing anelectrical cathode, the tool terminating in an electrolyte directingnozzle, comprising the steps of: directing the electrolyte from thecathode through the nozzle in a charged electrolyte stream toward and incontact with the anodic workpiece across an equilibrium gap;concurrently applying an electrical potential of sufficient intensitythrough the electrolyte stream so that the current passed between thecathode and the workpiece by the potential produces in the electrolytestream at least an incipient glow wherein the currentvoltagerelationship in the electrolyte is at least in the Kellogg region; whileat the same time charged electrolyte stream toward and in contact withthe anodic workpiece across an equilibrium gap; while at the same timeapplying between the cathode and the workpiece an electrical potential;contacting the stream of charged electrolyte with means to dissipate asufficient amount of the charge from the electrolyte stream to inhibitelectrolytic material removal from the workpiece at gaps greater thanthe equilibrium gap.

2. An electrolytic method for removing material from an anodic workpiecethrough the use of a hollow tool having a dielectric wall encasing anelectrical cathode, the tool terminating in an electrolyte directingnozzle, comprising the steps of:

immersing in an electrolyte bath that portion of the workpiece surfacefrom which material is to be removed;

positioning the tool and the workpiece one opposite the other;

contacting the cathode with an electrolyte;

directing the electrolyte from a cathode through the nozzle in a chargedelectrolyte stream toward and in contact with the anodic workpieceacross an equilibrium gap;

applying between the cathode and the workpiece an electrical potential;and

moving the cathode and workpiece one toward the other so that the streamof electrically charged electrolyte penetrates the bath into contactwith the workpiece across an equilibrium gap while passing electricalcurrent between the cathode and the workpiece through the electrolytestream in an amount sufiicient to produce at least an incipent glow inthe electrolyte stream contacting the workpiece wherein thecurrent-voltage relationship in the electrolyte is at least in theKellogg region but less than that amount of current which will produce aworkpieceenveloping vapor film which increases electrical resistancebetween the cathode and the workpiece sufi'icient to terminate removalof material from the workpiece.

3. The method of claim 2 in which the rate of moving the cathode andworkpiece one toward the other is the same as the linear rate ofmaterial removal across the equilibrium gap.

4. The method of claim 2 for producing cavities of up to 0.05" as amaximum width dimension, in which:

the potential applied between the cathode and workpiece is 300-1200volts with the current flow not exceeding 4 amps.

5. The method of claim 4 for use with an electrically conductiveworkpiece based on nickel in which:

the potential is maintained within the range of about 400-800 volts withthe current flow not exceeding about 2 amps;

the temperature of the electrolyte stream and the electrolyte bath beingabout 70-200 F.; and

the charged electrolyte stream being directed under a pressure of about40-60 p.s.i.

6. The method of claim 2 wherein a plurality of electrolyte directingnozzles are used.

References Cited UNITED STATES PATENTS 2,741,594 4/1956 Bowersett204-143 2,767,137 10/1956 Evers 204-143 3,067,114 12/1962 Tileyetal204-143 3,085,055 4/1963 Bradley 204 143 3,184,399 5/1965 Schnable204-143 3,267,014 8/1966 Sanders 204-443 3,357,912 12/1967 Inoue 204-224OTHER REFERENCES Schnable et al., Electrochemical Technology, July-August 1963, pp. 203-211.

Kellogg, J. of the Electrochemical Soc., vol. 97, No. 4, pp. 133-142,April 1950.

ROBERT K. MIHALEK, Primary Examiner.

1. AN ELECTROLYTIC METHOD FOR REMOVING MATERIAL FROM AN ANODIC WORKPIECETHROUGH THE USE OF A HOLLOW TOOL HAVING A DIELECTRIC WALL ENCASING ANELECTRICAL CATHODE, THE TOOL TERMINATING IN AN ELECTROLYTE DIRECTINGNOZZLE, COMPRISING THE STEPS OF: DIRECTING THE ELECTROLYTE FROM THECATHODE THROUGH THE NOZZLE IN A CHARGED ELECTROLYTEE STREAM TOWARD ANDIN CONTACT WITH THE ANODIC WORKPIECE ACROSS AN EQUILIBRIUM GAP;CONCURRENTLY APPLYING AN ELECTRICAL POTENTIAL OF SUFFICIENT INTENSITYTHROUGH THE ELECTROLYTE STREAM SO THAT THE CURRENT PASSED BETWEEN THECATHODE AND THE WORKPIECE BY THE POTENTIAL PRODUCES IN THE ELECTROLYTESTREAM AT LEAST AN INCIPIENT GLOW WHEREIN THE CURRENTVOLTAGERELATIONSHIP IN THE ELECTROLYTE IS AT LEAST IN THE "KELLOGG REGION";WHILE AT THE SAME TIME CHARGED ELECTROLYTE STREAM TOWARD AND IN CONTACTWITH THE ANODIC WORKPIECE ACROSS AN EQUILIBRIUM GAP; WHILE AT THE SAMETIME APPLYING BETWEEN THE CATHODE AND THE WORKPIECE AN ELECTRICALPOTENTIAL; CONTACTING THE STREAM OF CHARGED ELECTROLYTE WITH MEANS TODISSIPATE A SUFFICIENT AMOUNT OF THE CHARGE FROM THE ELECTROLYTE STREAMTO INHIBIT ELECTROLYTIC MATERIAL REMOVAL FROM THE WORKPIECE AT GAPSGREATER THAN THE EQUILIBRIUM GAP.